1. Field of the Invention
The present invention relates to a process for working up a residue fraction by methanolysis, the residue fraction originating from the raw ester distillation of the DMT production process.
2. Discussion of the Background
Dimethyl terephthalate (DMT) is produced on a large scale in numerous plants around the world. DMT is an important starting compound for the preparation of polyesters. Polyesters have long been used in the preparation of fibers, films, including photographic films, magnetic tapes or bottles of polyethylene terephthalate, to name only a few.
The conventional Witten DMT process essentially comprises (cf. EP-B 0 464 046, DE-A 40 26 733) the process steps of
(1) Oxidation of para-xylene (p-X) and methyl para-toluate (p-TE) with downstream waste gas purification;
(2) Esterification of the reaction products from the oxidation with methanol;
(3) Separation of the raw ester formed into
a) a fraction which is recirculated to the oxidation, PA1 b) a raw DMT fraction containing more than 99% by weight of DMT and PA1 c) a high-boiling residue fraction including its work-up; and
(4) Purification of the raw DMT fraction, for example by washing, recrystallization or pure distillation.
It is also possible to prepare terephthalic acid from DMT-rich fractions by targeted hydrolysis.
The oxidation of a mixture of para-xylene (p-X) and methyl para-toluate (p-TE or pT ester) is generally carried out in the liquid phase using atmospheric oxygen in the presence of a heavy metal catalyst (DE-C 20 10 137) at a temperature of from about 140.degree. to 180.degree. C. and a pressure of from about 4 to 8 bar abs. The oxidation step results in a reaction mixture which contains predominantly monomethyl terephthalate (MMT), p-toluic acid (p-TA) and terephthalic acid (TA) dissolved or suspended in p-TE. This mixture is esterified with methanol at a temperature of from about 250.degree. to 280.degree. C. at a pressure of from 20 to 25 bar abs. The raw ester obtained is separated by distillation into a p-TE fraction, a raw DMT fraction and a high-boiling, catalytic-containing residue fraction. The p-TE fraction is recirculated to the oxidation and the raw DMT fraction is converted via subsequent purification steps into the desired product quality.
The residue fraction originating from the raw distillation is generally further treated by methanolysis. FIG. 1 shows the flow diagram of a conventional single-stage methanolysis. In the reaction distillation column (1.1), the residues (1.2) and superheated methanol vapour (1.3) are continuously introduced in countercurrent at atmospheric pressure. The bottom of the column is additionally heated by means of a heat-transfer oil (1.4). The methanolysis is carried out at a temperature of from 265.degree. to 280.degree. C. Part of the residues are converted into materials which can be reused in the process. Acids present in the residue fraction are esterified in the methanolysis, part of the high-boiling organic compounds are dissociated and organic products of value already present are separated from undesired organic compounds which can no longer be used. The valuable or useful materials thus obtained go together with excess methanol via the top of the column to the dephlegmator (1.5) and are subsequently recirculated to the process, i.e. to the oxidation (1.6). The bottom residue formed in the methanolysis is generally conveyed to catalyst recovery (1.7). It is found in practice that carbon deposits and blockages occur in the reaction column during methanolysis, resulting in more frequent and unplanned stoppages of the unit.
EP-B 0 464 046 discloses a two-stage methanolysis for working up the residue from the raw ester distillation. The first methanolysis stage essentially comprises a reactor with upstream heat exchanger and circulation system, with a distillation column arranged downstream of the top of the reactor. In contrast to the first stage, the second methanolysis stage comprises a reaction distillation column. In both stages, additional methanol in vapour form is fed into the respective residue circuit upstream of the heat exchangers, i.e. before entry into the respective methanolysis reactors. The operating temperatures for both methanolysis stages are more than 265.degree. C.; particularly in the second methanolysis stage. Furthermore, each methanolysis stage is operated as a circuit and only partial streams are replaced, with the throughput amounts of the residue fraction to be worked up naturally being comparatively small. In addition, the two-stage methanolysis requires a high investment and high maintenance cost for operation of the plant.